Structure, vascularization, and innervation of the mystacial pad of the rat as revealed by the lectin Griffonia simplicifolia.

The mystacial pad of the rat is endowed with rows of vibrissal follicle-sinus complexes (F-SCs) that receive a dense and rich variety of innervation, much of which is C fibers. Each F-SC consists of a follicle at the core of a spindle-shaped, encapsulated vascular sinus. Previous studies have shown that the B subunit of the lectin Griffonia simplicifolia (GSA I-B4) binds selectively to a subset of small neurons in the trigeminal ganglion and to a subset of C fibers preferentially distributed to inner lamina II and outer lamina III of nucleus caudalis in the brainstem trigeminal complex in the rat. These laminae are also a major site of termination for afferents in superficial vibrissal nerves (SVNs) that innervate the upper portion of F-SCs. To determine the peripheral distribution of the afferents that bind GSA I-B4, mystacial pads from rats were prepared for fluorescence microscopy with GSA I-B4 conjugated to rhodamine. At the neck of each F-SC, numerous circumferentially oriented bundles of fine-caliber axonal profiles were labeled in the inner conical body, which receives nearly all of its innervation from the SVNs. A sparse, random distribution of fine-caliber profiles from deep vibrissal nerves was labeled at the level of the cavernous sinus in the deep half of the F-SCs. GSA I-B4 also labeled a variety of nonneural structures. By binding to vascular linings, GSA I-B4 revealed a dense, highly organized capillary system within the mesenchymal sheath that forms the inner lining of the vascular sinuses. Thus each F-SC appears to have a closed capillary system within the open vascular sinus. Trabeculae within the lumen of the cavernous sinus were also revealed to span between the sinus capsule and the mesenchymal sheath only about midway along the length of the follicle instead of the entire deeper half, as was previously believed. in addition, GSA I-B4 bound to the surface of follicular cells preferentially in the superficial half of the F-SCs. Sweat glands within the intervibrissal fur and some cells within sebaceous glands in F-SCs were also labeled as well as their ducts. The potential functional implications of these various features are discussed.